I am not going to disagree a lot would need to change to make healing more resource based, but I do think you overstate the 'AFK healer' issue. Other action MMOs do a good job of making healers both DPS and heal while everyone is mostly dodging abilities and make healing feel non-trivial. This is not some unsolved design issue plauging more modern MMOs. In GW2 you can run out of healing and you don't 'go afk' when you top someone off. You don't even do that in WoW, I have played and mained healer in WoW for ages and the idea of a healer running out of mana and AFKing is sorta outrageous, for one running out of mana is unusual and indicates something else in the raid went wrong, and for another you have tools that do not use mana. It is fully possible to have a healing focused job with limited healing resources that does not ever go AFK. I mean Astro already kinda is that with bad mana tools and a lot of focus on oGCD healing.
You are contextualizing running out of mana or topping off on resource based healers as 'go afk' but in reality running out of mana in resource based MMOs is a fail state on par with the tank and off tank running out of HP, and it isn't hard to design a fight for more incidental unavoidable damage to exist without changing the core of FFXIV. It isn't something that just inevitably happens unless the healer is bad.
I DO agree FFXIV probably shouldn't go the pure 'mana chugging healer' route. There is a reason Astro feels so good, and that is because it has healer resources that are limited but don't limit your APM: High cooldowns and a lack of focus on DOT which makes efficiently healing at specific times with ED a delight and viscerally rewarding. A big issue with FFXIV healers is that a lot of their healing is very efficient GCD heals that lack real weaknesses because every healer is sorta designed to be able to bring the entire team to full health with their AOE heal spam multiple times without even taking a break. Because every healer has an AOE heal nuke, a single target mantinence heal no one should use, and a single target serious heal that is infinitely spammable, healing is always somewhat trivial, and thus how much healing output you have and how efficient it is is also trivial: The fact that the WHM has regens (which increase mana and action efficiency) and has mana efficient bursts doesn't matter because everyone is already so good at sustaining every aspect of healing at all times that being slightly better doesn't matter.
I am not saying 'DPS focused healers with 100% uptime are bad and shouldn't exist.' They definitely should keep at least one healer like that. Cat druid healer in WoW was great for that reason and I am eternally sad its gone. But you should definitely try to empathize more with the people who played healers in other MMOs and found healer content in FFXIV is... really just quite terrible because it doesn't focus at all on the support/team monitoring aspects of play they are used to. Like the fact that WoW does healing objectively better than FFXIV is troubling, especially as we get more WoW refugees. No one who enjoys Priest in WoW, Water elementalist in GW2, Operative in SWTOR, Logi in EvE, ect, is going to enjoy the healers in FFXIV purely because of the current philosophy of 'minimize focus on healing resources' which is objectively not a good thing long term for the health of the game.
That said, we probably agree more than we disagree in the sense healing ATM feels very unrewarding. So I can, again, get behind this idea of voicing discontent. I just want to re-emphasize that 'I want to play healer to focus on healing well, not tossing out an overly basic DPS rotation, even in roulette content' is not an imagined or even uncontroversial desire, which some people seem to be implying is the case.